1. Field of the Invention
The present invention relates to an electromechanical transducer and a method for manufacturing the electromechanical transducer.
2. Description of the Related Art
Capacitive electromechanical transducers manufactured using a micromachining step have been actively investigated in recent years. A typical capacitive electromechanical transducer includes a lower electrode as a first electrode, a vibrating membrane supported so as to maintain a certain distance between the vibrating membrane and the lower electrode, and an upper electrode disposed on a surface of the vibrating membrane. Such a capacitive electromechanical transducer is used as, for example, a capacitive micromachined ultrasonic transducer (CMUT).
A CMUT can transmit and receive ultrasonic waves using a lightweight vibrating membrane and has good wideband characteristics in the liquid and air. Thus, a CMUT receives attention as an up-and-coming technology.
The operating principles of a CMUT will now be described. In the transmission of ultrasonic waves, low AC voltage is applied between the lower electrode and the upper electrode while being superposed on DC bias voltage. As a result, the vibrating membrane vibrates to generate ultrasonic waves. In the reception of ultrasonic waves, since the vibrating membrane is transformed due to ultrasonic waves, a signal is detected from the capacitance change between the lower electrode and the upper electrode caused by the transformation.
The sensitivity of a capacitive electromechanical transducer such as a CMUT depends on, for example, the distance (gap) between the electrodes, the rigidity of the vibrating membrane, an area of the electrode, and DC bias voltage. The higher DC bias voltage is, the higher the sensitivity becomes. In the case where a capacitive electromechanical transducer contacts the surface of a living body or living tissues in the body, an insulating protective layer needs to be disposed on a surface of the capacitive electromechanical transducer (refer to Journal of Micromechanics and Microengineering, Vol. 17, 2007, pp 994-1001).